Exhaust pressure is high when the exhaust valve opens to allow combustion gases into the exhaust manifold. However, once the valve closes, exhaust pressure drops below atmospheric pressure to create a vacuum in the exhaust manifold. This explains why exhaust pressure rapidly pulsates above and below atmospheric pressure.
The PAIR system promotes HC and CO oxidation by adding additional oxygen into the exhaust manifold during cold engine operation and deceleration (when very specific parameters are met). These operating conditions typically produce higher levels of HC and CO emissions.
This system simply provides a controlled air passage between atmosphere and the exhaust manifold. Whenever exhaust manifold pressure drops below atmospheric pressure, fresh air from the high pressure zone (atmosphere) flows through the system and enters the exhaust manifold where it promotes emission oxidation.
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